Spread spectrum systems for the transmission and reception of signals, particularly in wireless transmission, has been established for many years. A general description of such systems is given, for example in ‘Spread Spectrum Systems’, by Robert C Dixon, published by John Wiley and Sons (Second Edition, 1984). Spread spectrum techniques have proved versatile and effective in discriminating against both broadband noise and specific interfering signals, since the correlation process (otherwise called despreading) in accordance with the original spreading will spread an interfering signal and thereby reduce the amplitude of its frequency components relative to the components of the desired and despread signal
Nevertheless, spread spectrum receivers are not proof against interference and where there is a strong interfering signal within the effective bandwidth of the spread spectrum signal, a substantial error rate is likely The effects of a single interfering signal can to some extent be countered by employing redundancy in the transmitted signal, though that is not a desirable solution in general except possibly for systems where a high degree of security at the cost of a lower effective information rate is desirable.
There are three general techniques for producing spread spectrum signalling, namely the modulation of a carrier by a digital code sequence, known as ‘direct sequence’ modulation, the sweeping of a carrier over a wideband during a given pulse interval, often called ‘chirp’ modulation, and carrier frequency shifting in discrete increments in a pattern dictated by a code sequence, usually termed ‘frequency-hopping’ The present invention relates to systems which include a receiver for a first spread spectrum system (particularly a ‘direct sequence’ system) and apparatus, which may be a transmitter or receiver or transceiver, which can transmit and/or receive a frequency-hopped spread spectrum signal that for at least part of the time is within the spread spectrum of the signal intended for reception by the first receiver.
In a pertinent example to which the invention relates, a radio receiving apparatus may include a first receiver (which may form part of a transceiver) which is intended to receive a spread spectrum signal produced in accordance with IEEE Standard 802.11b. The apparatus also includes either a transmitter or receiver (and more usually both) which operates according to a frequency-hopped spread spectrum system, for example that which is commercially termed a ‘Bluetooth’ system
A receiver of this nature is provided in order to be able to communicate with devices operating with two different spread spectrum transmission standards
In this particular example, the Standard prescribed by IEEE 802.11b specifies a spread spectrum transmission that effectively occupies a bandwidth of 22 MHz within the range 2400 to 2480 MHz The ‘Bluetooth’ transmission is a frequency-hopped transmission which occupies a 1 MHz band for a short period of time before being frequency-hopped The total allocated bandwidth is of the order of 80 MHz The usable spectrum of a 802.11b signal only employs 22 MHz of this bandwidth However, the ‘Bluetooth’ signal will hop in to the same frequency band as the 802.11b signal about one quarter of the time.
An 802.11b receiver will accept the corresponding spread spectrum signal and despread it to recover the original signal During this process, any narrow band interfering signal will be spread out and the interference is reduced in level. In normal operation, the ratio Es/No, where Es in the energy per signal and No is the normalised noise, is about 8 decibels for an 11 Mbps data rate. If the noise (represented by the ‘Bluetooth’ signal while it is within the bandwidth of the 802.11b spread spectrum signal) is more than 8 dB above the 802.11b spread signal, as may well be expected, then considerable interference will be experienced
The present invention is therefore concerned with the reduction of interference in a spread spectrum transmission wherein the interference arises from a frequency-hopped signal which is received or transmitted locally.